In the prior art, time delay relays in which a PTC heater is located in heat transfer relationship with a bimetal snap disc are well known. Typical of such devices is the one shown in U.S. Pat. No. 3,858,139 in which a bimetal disc is located in a cup-shaped member to which the PTC heater is adhesively attached. In the device shown in this patent, one electrical connection to the heater is made by a spring blade biased against the heater and the other connection is made by a second spring blade biased into engagement with the cup-shaped member. The bimetal snap disc is arranged such that it moves a yoke havig two projections thereon which each actuate a switch when the yoke is moved by the bimetal disc.
A number of problems are associated with the aforementioned prior art thermal relay device. Firstly, the thermal coupling between the PTC heater and the bimetal snap disc is dependent on ambient temperature conditions to a greater extent than is desired because of the terminal extending from outside the housing to make electrical contact to the cup-shaped member. Secondly, the thermal coupling between the PTC heater and the bimetal snap disc in the aforementioned prior art device is dependent on the position of the bimetal snap disc to a certain extent. Thirdly, the yoke structure used in the prior art device to actuate a pair of switches is subject to substantial variations in size, and these size variations will result in differences in the operating temperature of the two switches.